Cognitive Radio (CR) is considered as a type of radio in which communications systems are aware of their environment as well as their internal state and may make decisions about their radio operating behavior based on that information and predefined objectives. A CR system generally consists of a plurality of primary users (PUs), also referred to as licensed users (LUs), and a plurality of secondary users. Examples of PUs may include TV stations or wireless microphones in TV bands. The secondary users are unlicensed CR users and are able to sense the spectrum utilization of the PUs.
In CR systems, a communication can be established when a CR transmitter sends its signal without interfering with PUs, and in the meantime, a CR receiver can decode such signals under certain interference from PUs, in other words, a communication opportunity appears when CR users can transmit legally and receive reliably. CR users can utilize licensed spectrum bands when they do not cause interference to PUs. A time when CR users can transmit without causing interference to the primary users is called a transmission opportunity or a spectrum opportunity. Clearly, the ability to sense the spectrum to detect spectrum opportunities is crucial to CR performance and has been the subject of considerable research.
The basic concept of CR is to allow unlicensed CR users to use licensed spectrum bands (also referred to as licensed frequency bands) as long as they do not cause interference to PUs. Therefore, CR users must be able to identify and use spectrum bands that are not being used by primary users. In practice, the available spectrum bands for CR users may vary with time and location. When a CR user has information to transmit to multiple CR receivers, resources corresponding to transmission opportunities may need to be allocated to the various CR receivers so that the information may be transmitted to the CR receivers while maintaining performance requirements and goals.